Conductance stability and Na+ interaction with Shab K+ channels under low K+ conditions

K(+) ions exert a structural effect that brings stability to K(+) selective pores. Thus, upon bathing Shab channels in 0 K(+) solutions the ion conductance, G(K), irreversibly collapses. Related to this, studies with isolated KcsA channels have suggested that there is a transition [K(+)] around which the pore takes one of two conformations, either the low (non-conducting) or high K(+) (conducting) crystal structures. We examined this premise by looking at the K(+)-dependency of G(K) stability of Shab channels within the cell membrane environment. We found that: K(+) effect on G(K) stability is highly asymmetrical, and that as internal K(+) is replaced by Na(+) G(K) drops in a way that suggests a transition internal [K(+)]. Additionally, we found that external permeant ions inhibit G(K) drop with a potency that differs from the global selectivity-sequence of K(+) pores; the non-permeant TEA inhibited G(K) drop in a K(+)-dependent manner. Upon lowering internal [K(+)] we observed an influx of Na(+) at negative potentials. Na(+) influx was halted by physiological external [K(+)], which also restored G(K) stability. Hyperpolarized potentials afforded G(K) stability but, as expected, do not restore G(K) selectivity. For completeness, Na(+) interaction with Shab was also assessed at depolarized potentials by looking at Na block followed by permeation (pore unblock) at positive potentials, in solutions approaching the 0 K(+) limit. The stabilizing effect of negative potentials along with the non-parallel variation of Na(+) permeability and conductance-stability herein reported, show that pore stability and selectivity, although related, are not strictly coupled.